A trapped particle acts like a mass on a spring, but the restoring forces are provided by electrodynamics. Using the invention, exquisitely machined physical mechanisms can be replaced by carefully tuned mechanical physics, yielding inertial sensors that are simpler to build yet exhibit superior performance because their operating parameters can be dynamically controlled.
An accelerometer can be most simply described as a mass on a spring, at equilibrium with the local acceleration field, combined with some means to read out the equilibrium position and thereby infer the applied acceleration. This simplified model of an accelerometer is employed in most of the exposition to follow.
For many years the market for inertial measurement devices was restricted to defense and aerospace applications in which the devices were optimized for high accuracy and low bias. The production of these devices was limited, and both their price and complexity remained high.
The first accurate, mass-manufactured accelerometers were micromachined interferometric (“MEMS”) devices that found application as automotive collision detectors. See, for example, the Analog Devices ADXL202/ADXL210 product datasheet, “Low Cost±2 g/±10 g Dual Axis iMEMS Accelerometers with Digital Output, (Analog Devices, Norwood, Mass., 1999). Since then, new applications and markets for accelerometers have begun to emerge, but none yet have a volume comparable to the automotive market. One might reasonably argue that other applications have not yet emerged because available inertial sensors still do not meet niche requirements including: price per degree of freedom, physical sensor size, sensitivity, bandwidth, and drift.
The present invention employs particle traps to provide accelerometers of a new kind. Particle traps are key to many of today's most sensitive and accurate metrological techniques. They are commonly used as mass balances and spectrometers, and also provide the isolation and control necessary to manipulate atoms, ions, and electrons singly or in stable ensembles for arbitrarily long intervals. The present invention provides a new use for such suspended structures as inertial sensors.
The present invention takes the form of an inertial sensor consisting of an electrodynamic trap for suspending one or more charged particles and a readout device for measuring variations in the position or motion of the particles when the trap is subjected to acceleration forces. Particle position and/or motion may be measured by optical interferometry, optical leverage, resonant electric field absorption, or by producing an image of the particle motion and processing the image data to obtain values representing the acceleration forces on the trap in one to six degrees of freedom. The electrodynamic trap employs electrodes to which a time-varying potential are applied to produce a quadupole field that constrains the charged particles to a specific location between said electrodes by a substantially linear, tunable restoring force.
A practical, manufacturable sensor requires a simple, accurate readout technique such as optical interferometry, optical leverage, or resonant electric field absorption. However, to allow flexibility in construction and characterization of our sensors, the preferred embodiment which will be described below uses metrological techniques based on video microscopy and particle tracking by image processing.
Since the prototype accelerometers are large structures with many mechanical resonances, characterization is performed at known applied static accelerations by rotating the trap in the earth's gravitational field. To demonstrate the tunability of the trap, operating parameters such as guiding potential scale and frequency are also varied. Since the image of a particle on the sensor covers several pixels, particle positions may be determined to sub-pixel resolution by simple interpolation. Although video microscopy affords a high pixel bandwidth (on the order of MHz), the temporal resolution of image features is limited to the frame rate (tens of Hz). While this does not greatly limit long-term drift or bias measurements, we would like to perform measurements of the noise spectral density over a wider band. Accordingly, a second experiment is devised to provide a readout of a particle's motional noise from the reflected intensity profile as it intersects the waist of a focused beam. The measurement is calibrated by the application of small static accelerations.